It is generally known that transmitter/receiver (transceiver) and other communication devices can obtain their power from a host device. Such an arrangement is described in U.S. Pat. No. 5,905,758, which is owned by the assignee of the present invention. In that arrangement, power is obtained through a serial communications port at the host device. The signals from the host device at the serial port include both EIA-232 data as well as device control signals. The EIA-232 signal standard specifies a voltage greater than or equal to ±5 volts DC, with an output resistance of 300 ohms. Many host devices include an internal +5 volt supply rail, which powers the EIA-232 driver IC (integrated circuit). The driver IC will typically include a conventional internal charge pump to produce ± volt internal voltage rails. Those voltages drive the EIA-232 signal. Due to circuit inefficiencies, however, the output voltage of the EIA-232 signal will be ±9.5 volts DC. However, most transceivers do not require that high a voltage level. In fact, many transceivers including those disclosed in the '758 patent, are easily powered with voltages down to 5 volts DC.
Many newer host devices, however, have an internal voltage rail of less than 5 volts DC, e.g. 3.3 volts DC for the drive IC. A conventional charge pump would bring these voltages up to ±6.6 volts DC. With an output resistance of 300 ohms and a typical load of 10 mA, the output voltage of the EIA-232 signal would be only 3.1 V, which is too low for many transceivers, including the '758 transceiver.
Further, the digital electronics industry has a present standard of 3.3 volts DC for IC circuits (many new electronic circuits use 3.3 volts), and the design trend is toward even lower IC voltages. Additional charge pump circuitry would be necessary, however, in the device IC's internal circuitry to produce the required voltage rails for the EIA-232 signal, if the internal supply goes lower than 3.3 volts.
In addition, in some cases, only positive or only negative EIA-232 signals are present. Many host device powered transceivers, however cannot operate with only positive or negative voltage EIA-232 signals. It would hence be desirable for a transceiver or similar communication device to be able to use power from its host device provided at low voltages, i.e. 3.3 volts DC and below, to produce the desired ± voltage rails, as well as to produce both plus and minus supply voltages when only one EIA-232 voltage is present or the EIA-232 voltage switches between positive and negative values.